Recent years have seen progress in the development and practical implementation of display devices (hereafter referred to as organic EL display devices) using organic EL elements. Generally, an organic EL display device includes (i) a display unit having, arranged in a matrix, pixel circuits each having an organic EL element, and (ii) a drive circuit for driving the display unit.
A fundamental pixel circuit used in an active-matrix organic EL display device is configured to include an organic EL element, a selection switching transistor, a capacitor, and a drive transistor. In such a pixel circuit, data voltage is held in the capacitor by, first, placing the selection switching transistor connected to the signal line in a conducting state, storing the data voltage corresponding to the luminance of the pixel into the capacitor from the signal line, and subsequently placing the selection switching transistor in a non-conducting state. Next, a current commensurate in size to the voltage held in the capacitor is supplied from the drive transistor to the organic EL element, and the organic EL element emits light at a luminance corresponding to the data voltage, according to the current supplied from the drive transistor.
With respect to such fundamental pixel circuits, there have been various proposals for pixel circuits provided with a configuration for causing an organic EL element to emit light at a luminance that more precisely corresponds to the data voltage, and for methods of controlling the same (for example, Patent Literature (PTL) 1).
FIG. 20 is a circuit diagram illustrating a conventional pixel circuit 90 disclosed in PTL 1.
The pixel circuit 90 includes a drive transistor TD, switching transistors T1 to T3, a capacitor Cs, and an organic EL element EL.
The pixel circuit 90 is supplied with control signals from the scanning line drive circuit 4 via signal lines SCAN and MERGE, and is supplied a with data voltage corresponding to luminance, from the signal line drive circuit 5 via a data line DATA. Furthermore, the pixel circuit 90 is supplied with positive and negative power source voltage used in the light-emission of the organic EL element EL, from a power source circuit not shown in the figure via power source lines VDD and VSS, and supplied with a reference voltage via a reference voltage line Vref.
Although complex voltage change caused by voltage drops occur at the points where the power source lines VDD and VSS, which supply current to the organic EL element EL, are connected to the pixel circuit 90, a steady voltage drop rarely occurs at the reference voltage line Vref which does not supply direct current.
The pixel circuit 90 having the above configuration operates in the subsequent manner according to the control signal supplied. It should be noted that, in the subsequent description, the operation of applying a voltage A to one end of the capacitor and a voltage B to the other end of the capacitor, and holding in the capacitor a voltage (A-B) which is the difference between voltage A and voltage B is expressed as holding voltage A in the capacitor with reference to voltage B. This expression shall be used throughout the Specification.
First, the switching transistor T2 is placed in the non-conducting state, and, with the capacitor Cs being electrically separated from the current path within the pixel, the switching transistors T1 and T3 are placed in the conducting state. The capacitor Cs holds the data voltage with reference to the reference voltage.
At this time, the voltage held in the capacitor Cs is completely unaffected by the change in power source voltage. Next, the switching transistors T1 and T3 are placed in the non-conducting state, the switching transistors T2 is placed in the conducting state, and the voltage held in the capacitor Cs is applied across the gate terminal and the source terminal of the drive transistor TD.
As a result, since the drive transistor TD supplies the organic EL element EL with a current that is in accordance with only the data voltage, the organic EL element EL emits light at a precise luminance corresponding to the data voltage.